PKENTISS : THE OTOCYST OF DECAPOD CRUSTACEA. 241 



brain. In the case of adult Brachyiira, however, there are no otoliths 

 in contact with the hairs of the otocyst, consequently the effect of 

 gravity, if not entirely null, must be at least greatly lessened, unless 

 indeed the hairs are so differentiated as to be themselves stimulated 

 by it. 



Bethe ('97), acting on the idea that in tilting the animal the differ- 

 ence in the pressure of the water might affect the hairs of the otocysts, 

 placed crabs under very high pressures where the slight difference brought 

 about by tilting would be practically eliminated. But he found that all 

 the phenomena of equilibration still persisted. 



It is probable that in the otocyst of Cai'cinus the thread hairs are the 

 most important sensory organs of the sac. The hook hairs, originally 

 in the larva attached to otoliths, later, with the loss of the sand granules, 

 lose much of their functional activity ; the third group of hairs can- 

 not be of great importance, as I could not demonstrate satisfactorily 

 their nerve connections, and their structui'e alone is such as to preclude 

 their being affected by very delicate stimuli. The thread hairs, how- 

 ever, in both structure and position are fitted for the fulfillment of such 

 a function as has been ascribed to them. Tlie shaft is long, attenuate, 

 only slightly fringed at the tip, and attached at the base by a very thin 

 membrane, which allows free movement to the rigid shaft about this 

 region as upon a joint. I have observed in studying freshly dissected 

 otocysts that a slight tilting of the watch glass in which they were con- 

 tained caused these hairs to sway extensively. 



From Clark's experiments and my own, it was apparent that upon 

 rotation in a horizontal plane, there was little or no compensatory 

 movement of the eyestalks, and that when there is such a reaction, the 

 angle of compensation is not maintained, but the eyes return at once to 

 tlieir original positions. Also, on rotation about the animal's lateral 

 axis, the angle of compensation is not as great, when the rotation is 

 rapid and jerky, as when performed slowly and smoothly. These two 

 facts preclude the possibility of the hairs being affected by movements of 

 the fluid surrounding them, at least to any great extent. For if they 

 were so affected, the angle of compensation should be the same, in what- 

 ever plane the animals are rotated, and the position of the eyestalks 

 should be in every case maintained by compensation movements. 



There still remain two ways in which the hairs may be so affected as 

 to bring about nervous stimulus. Either they may be lighter than the 

 surrounding fluid, and consequently tend always to float erect, no matter 

 what position the otocyst may take relative to them j or they may be 



